With an increase in the recording density of magnetic disks, the distance between the magnetic disk serving as a recording medium and the head for use in recording of information or playback has become almost nil close to contact therebetween. The magnetic disk is provided over the surface thereof with a carbon protective film or lubricant film for the purpose of diminishing abrasion due to the contact or sliding of the head thereon or preventing contamination of the disk surface.
The carbon protective film is produced generally by the sputtering process or CVD process. Since the disk surface is protected with the two films, i.e., the carbon protective film and the lubricant film thereover, the interaction between the carbon protective film and the lubricant is important.
The lubricants generally in use are perfluoropolyethers having functional groups. Examples of functional groups are hydroxyl, amino and cyclophosphazene group. More specific examples of lubricants include Fomblin ZTETRAOL manufactured by Solvay Solexis Inc. and having hydroxyl, and PHOSFAROL A20H, product of MORESCO Corporation and having both hydroxyl and cyclophosphazene groups.
Fomblin ZTETRAOL exhibits good properties to adhere to the disk due to the presence of the hydroxyl at opposite terminals of the molecule, does not spatter even when the disk rotates at a high speed and is capable of retaining the form of a lubricant film. However, this perfluoropolyether compound is low in resistance to Lewis acids, reacts with Al2O3 in the component of the head and becomes cleaved in its main chain (see, for example, Nonpatent Literature 1). When this cleavage progresses, the compound becomes lower in molecular weight, eventually vaporizing off from the magnetic disk surface and failing to retain the form of a lubricant film in systems involving contact or sliding movement between the head and disk.
On the other hand, PHOSFAROL A20H has at one terminal of the molecule a hydroxyl group contributing to the adhesion to the disk and at the other terminal a cyclophosphazene group for inhibiting the cleavage of the perfluoropolyether main chain with Lewis acids. Accordingly, the lubricant does not spatter even if the disk is in high-speed rotation, the compound is inhibited from decomposing with the Al2O3 in the component of the head and the lubricant is capable of retaining the film form. However, if the carbon protective layer under the lubricant lowers in surface polarity, the number of lubricant molecules adsorbing to the ground layer through a hydrogen bond reduces because of the presence of a single hydrogen group, entailing the likelihood that the lubricant will spatter during high-speed rotation.
A straight-chain perfluoropolyether compound has been disclosed which is improved in adhering properties over A20H and which has at one terminal of the molecule a plurality of hydroxyl groups and at the other terminal a cyclophosphazene group (Patent Literature 1). It has been found that the disclosed compound exhibits higher adhering properties than A20H and ZTETRAOL, whereas compounds are expected which are satisfactory in both adhering properties and decomposition resistance.    [Nonpatent Literature 1] Macromolecules, 1992, Vol. 25, pp. 6791-6799    [Patent Literature 1] WO2006/009057
An object of the present invention is to provide a stable compound which remains free of decomposition even when brought into contact with the head and which exhibits good adhering properties on the disk surface.